1. Field of the Invention
The present invention relates generally to variable compression ratio internal combustion engines. More particularly, the invention relates to a method and system for determining the pressure of air within an intake manifold (xe2x80x9cmanifold pressurexe2x80x9d) of a variable compression ratio internal combustion engine.
2. Background Art
The xe2x80x9ccompression ratioxe2x80x9d of an internal combustion engine is defined as the ratio of the cylinder volume when the piston is at bottom-dead-center (BDC) to the cylinder volume when the piston is at top-dead-center (TDC). Generally, the higher the compression ratio, the higher the thermal efficiency and fuel economy of the internal combustion engine. So-called xe2x80x9cvariable compression ratioxe2x80x9d internal combustion engines have been developed, for example, having higher compression ratios during low load conditions and lower compression ratios during high load conditions. Various techniques have been disclosed for varying compression ratio, including for example, using xe2x80x9csub-chambers and xe2x80x9csub-pistonsxe2x80x9d to vary the volume of a cylinder, see for example patents U.S. Pat. No. 4,246,873 and U.S. Pat. No. 4,286,552; varying the actual dimensions of all or a portion of a piston attached to a fixed length connecting rod, see U.S. Pat. No. 5,865,092; varying the actual length of the connecting rod itself, see U.S. Pat. Nos. 5,724,863 and 5,146,879; and using eccentric rings or bushings either at the lower xe2x80x9clargexe2x80x9d end of a connecting rod or the upper xe2x80x9csmallxe2x80x9d end of the connecting rod for varying the length of the connecting rod or height of the reciprocating piston. See U.S. Pat. No. 5,562,068, U.S. Pat. No. 5,960,750, U.S. Pat. No. 5,417,185 and Japanese Publication JP-03092552.
As with conventional internal combustion engines, it is vitally important for a number of reasons to be able to accurately estimate or infer the air pressure within the intake manifold of a variable compression ratio internal combustion engine. Manifold pressure estimates are used, for example, to operate intake manifold filling models, and to properly control an electronic throttle, to name but two functions.
The inventor herein has recognized the need to accurately determine the manifold pressure as a function of a selected engine compression ratio in order to ensure optimal control and performance of the engine and the vehicle systems.
A method is provided for operating a variable compression ratio internal combustion engine. The method includes the steps of determining a compression ratio operating state of the variable compression ratio internal combustion engine, and inferring the engine""s intake manifold pressure based at least in part on the compression ratio operating state of the engine. For example, in accordance with the present invention, manifold pressure can be inferred by first determining the engine speed, engine load, and the current compression ratio operating state of the engine. The manifold pressure is determined by first extracting an intercept pressure value from an intercept lookup table as a function of engine speed and compression ratio. This value is summed with a slope pressure value determined as the product of engine load and a slope selected from a slope lookup table as a function of engine speed and compression ratio. If the compression ratio is subsequently changed, a new value for manifold pressure may be extracted from a second lookup table using the same inputs, with the exception of compression ratio, as were used with the first table. Alternatively, one or more scalar values may be applied to the appropriate value extracted from the baseline lookup table.
Advantageously, the methods described herein allow for improved estimates of manifold pressure that can be used to optimize scheduling of compression ratio operating states in a variable compression ratio internal combustion engine. The methods disclosed herein are useful for optimizing the fuel economy benefits of the engine, while at the same time improving control and performance of a corresponding motor vehicle.
In accordance with a related aspect of the present invention, a corresponding system is provided for operating a variable compression ratio internal combustion engine. The system includes a compression ratio setting apparatus for configuring the engine in selected ones of the compression ratio operating states, and a controller in communication with the sensors and the compression ratio apparatus, the controller comprising computer program means for inferring manifold pressure based at least in part on the compression ratio operating state of the engine. A system in accordance with a preferred embodiment further comprises a sensor coupled to the engine for generating a signal representative of engine speed, a sensor system coupled to the engine for generating a signal representative of engine load; and computer program code and look-up tables for determining at least one predefined exhaust temperature based at least upon on the engine speed, the engine load, and the compression ratio operating state of the engine.
Further advantages, as well as objects and features of the invention will become apparent from the following detailed description of the invention taken in conjunction with the accompanying figures showing illustrative embodiments of the invention.